Performance of Alumina Membranes from New Nanosynthesis in Ultrafiltration and Nanofiltration

Starting from simple inorganic or organic raw materials in aqueous or alcoholic solvents, a synthesis process has been developed for (i) the preparation of sol–gel‐derived alumina (Al ₂ O ₃ ) membranes or (ii) the manufacture of membranes by the deposition of precalcined nanocorundum (α‐Al ₂ O ₃ ) powders. Because of the equiaxed shape of the synthesized constituent particles, the permeability of the membranes is a factor of 3–5 greater than in previous products. In contrast to the case for known porous transitional Al ₂ O ₃ , the equiaxed shape of the particles in this case is enabled for any of the transitional phases and for the final corundum by a synthesis method that avoids the intermediate crystallization of platelet‐shaped boehmite. By this means, nanoporous Al ₂ O ₃ components with pore sizes of ∼1 nm or up to 5 nm can be derived preferentially by sol–gel processing, whereas both sol–gel and nanopowder techniques are used to manufacture ultrafiltration α‐Al ₂ O ₃ membranes with a high chemical and thermal stability and with average pore sizes of 10–60 nm. Compared with the sol–gel approach, powder processing decreases the drying and sintering shrinkage and, thus, the frequency and size of flaws, improving the cutoff characteristics of the membranes. For catalytic applications, the porous ceramic microstructures also can be applied on larger metal substrates.